1. Field of the Invention Technology
The present invention relates to microphone output signal levels, and more specifically, to the calibration thereof to a desired level. When output levels of different microphones are compared, it is assumed that the acoustical excitations thereof are identical. Manufacturers supply microphones having output levels varying around a specified mean value. For the often-used back-electret microphones, such tolerances are ±4 dB. Consequently, the output levels of such microphones may show a difference of up to 8 dB. Microphones with tolerances of ±2 dB are sometimes available. These, however, are more expensive.
2. Description of the Related Art
A usual approach for gain calibration of a microphone is carried out in an anechoic chamber, i.e., a chamber without reflections or reverberation. A loudspeaker is placed in front of the microphone (at an angle of 0°) inside the anechoic chamber. The loudspeaker plays a noise sequence at a known power level and the power of the microphone response is measured. Subsequently, an adjustable gain is set.
Further an audio processing arrangement is disclosed in International Patent Application No. WO 99/27522. According to this prior art reference, filtered sum and weighted sum beamforming are developed for maximizing power at the output. Filtered sum beamforming (FSB) makes the direct contributions maximally coherent upon adding thereof.
With multi-microphone algorithms such as beamforming, it is very important to sort the microphones during production to obtain sets with level differences within the required tolerances.
Moreover, with some multi-microphones systems, the consumer may buy additional microphones later in time, which will also have to be calibrated before installation.